1. Field of the Invention
The present invention relates to the design of ferrite cores for inductive proximity sensors of the ECKO type.
The purpose of the ferrite core in these sensors is to concentrate and focus the alternating magnetic field produced by a current-carrying winding. The magnetic flux is specifically aimed at the sensor's metal target, and since the sensing operation is dependent on the generation of eddy currents within the metal target, the range at which the target can be detected is a direct function of how optimally the magnetic flux is cut by the target. In a preferred embodiment the target is a circular disk, just as is used in industry for standardization procedures.
2. Description of Related Art
Proximity sensors are devices used to detect and register the presence of specific objects depending on their variety or principle of operation. They may be based on a number of principles; capacitance, eddy current, photoelectricity, or even the Hall effect. Depending on the principle of operation, each class of sensor will detect a certain class of objects (targets) only.
In particular, inductive proximity sensors operate on the principle of eddy currents. In view of this, they are capable of detecting and registering the presence of metals only. They are used extensively in industry for a very wide variety of non-contact sensing operations; from controlling the movement of individual parts of complex robots to other operations like counting metal cans on a conveyor belt.
Inductive proximity sensors consist of a coil wire supporting high frequency current. This high frequency current gives rise to a magnetic field oscillating at the same frequency. The oscillating magnetic field induces eddy currents in the metal target which causes damping of the oscillator circuit feeding the sensor's winding. The damped oscillation is detected by associated electronics--a Schmitt trigger circuit and additional chatter-prevention electronics. Because of this mode of operation, inductive proximity sensor technology has acquired the term ECKO technology, which stands for Eddy Current Killed Oscillator.
The following is a brief overview of different techniques that have been implemented in one form or another in the design of proximity sensors of the inductive type. It must be made clear that the design of all proximity sensors is a twofold problem; first, the specific principle on which the transducer operates must be decided upon. Secondly, the means by which the detector acquires and processes the information gathered by the transducer must also be conceived.
In what follows, various transducers and their associated detectors operating on the inductive principle are reviewed.
By far, the most widely used method of inductive sensing is the ECKO variation. As indicated earlier, this abbreviation stands for Eddy Current Killed Oscillator. The principle on which this operates is discussed in detail later. Sensors based on this mode of operation have a sensing range of up to about 50 mm for a standard 30 mm diameter size with the average sensing distance for ferrous metals being in the neighborhood of 15 mm.
One method, the inductance divider sensor, consists of a sensor inductor connected in series with an identical reference inductor between input and ground to form an inductive divider network. The sensor inductor is positioned so that its inductance can be varied in response to the proximity of a target while the inductance of the reference inductor remains constant. Voltage pulses are periodically applied to the divider network from a source and divide across the reference and sensor inductors in the ratio of their respective inductances. A detector monitors the output of the divider and provides information on the proximity of the target. It should be understood that this method of detection is suitable for metallic targets only since non-metallic targets are incapable of altering flux linkage (and hence inductance).
Another scheme for detecting the spatial proximity of a ferrous object is the magnetic bridge proximity sensor. This configuration comprises a magnetic reluctance bridge formed from a combination of high and low permeability sections. There is a ring core flux gate magnetometer positioned to form the center reluctance path of the bridge in order to sense flux when the bridge is unbalanced. A direct current magnetic field generator is positioned along a line of symmetry of the bridge to provide magnetic flux within the bridge. When a ferrous object appears within sufficient proximity of one of the low permeability sections it unbalances the bridge and causes flux to be detected by the flux gate magnetometer. Additional circuitry is provided for generating a triggering signal when the magnetometer detects sufficient flux.
Yet another variation of proximity sensor operates by having a field-creating coil arrangement which produces a field having a field strength minimum. It also has a sensing coil positioned at that field strength minimum which is responsive to changes in position of that minimum. When a target is brought close enough to the field-creating coil arrangement, the position of the field strength minimum moves. The sensing coil registers this displacement of the minimum and thus information concerning the proximity of the target is ascertained.
A fourth method of implementing target detection is based on the alteration of the magnetic flux pattern of a permanent magnet by the presence of a ferrous target. A permanent magnet is positioned within a chassis so that the axis and poles of the flux field are substantially normal to the front of the chassis. A switch is also positioned along the front of the chassis near the magnet. This switch has contacts operable by the magnetic flux field of the permanent magnet. Therefore the contacts of the switch operate in a fashion directly related to the proximity of the target and thus, information about its proximity is obtained. The flux pattern produced by the permanent magnet is further focussed by a ferrite core.
It is clear from the examples briefly presented that the vast majority of inductive sensors are capable of detecting only ferromagnetic targets. There are not very many metals that are ferromagnetic and this is the primary reason for the widespread use of the ECKO principle; it is capable of detecting non ferrous objects. It has, however, the additional requirement that the target be able to support eddy currents, i.e., that the target be an electrical conductor. Of course, it is most responsive to ferrous metals.
There is thus a need for new ferrite core geometries for inductive proximity sensors where such geometries yield increased sensing ranges. The present invention is directed toward filling that need.